In preparing tapped holes for receiving screw threaded fasteners, it is well recognized that for each particular size screw thread, a drilled hole must be first provided in the material which is to be tapped. Taps permit little variation in operating conditions as the rate of metal removed per rpm is governed by the effective chamfer length, the tap speed and minor diameter of the product. If the tap drill is not the proper size, thread height may be inadequate, or the tapping torque required may increase sharply and result in unsatisfactory tapped holes or even tap breakage. It is, therefore, important to select the optimum size tap drill for use with each tap. After a tap is used to cut threads in the previously drilled hole, it also may be necessary to drill a clearance hole through a second material to enable a threaded fastener to be inserted through the clearance hole into engagement with the threaded hole. Thus for each tap size, it is frequently necessary to have two different drill sizes available, i.e., the tap drill and the clearance drill for each tap size. The general practice has been to provide separate tap and drill sets with the drill sets having twice the number of drills as the corresponding tap set. Moreover, such drill and tap sets provide no convenient means for correlating the proper tap drill and clearance drill for each tap. The user will generally make reference to various reference tables for guidance in the selection of the proper drill sizes to use with each tap size.
It is the principal object of this invention to provide a unitary tool holder combining a series of taps and drills in which indicia indicate the proper tap drill and clearance drill for use with each tap.
It is another object of this invention to provide a tool holder of the above type containing the tap drills of a given size which serve both as the tap drill for one tap and as the clearance drill for the next smaller tap in the successive series.
A further object of this invention is to provide a tap and drill set in a holder of the above type in which the number of drills is only two more than the number of taps.